I think I'd be looking at wanting all .. x ray, ultrasound and blood tests .. possibly in that order as if the X-ray and the ultrasound don't show anything up then possibly bloods will.
Ultrasounds are a weird things, and need someone with experience and skill to interpret the images, so make sure there's some at your vet with this 'skill' / experience. The first vet who ultra sounded Jennie didn't really know what she was looking at (she was young and only few years out of uni I reckon).
Pain is a peculiar thing. When Jennie had her disc problem... I could have sworn to you it was her stomach, she grumbled when you touched her, would sit and shake, pace around, pant excessively, though she wasn't off her food (she's never off her food
).
After a while my vet was convinced as well, but when surgery found nothing and she took a turn for the worse it was obviously her back, and referral to a specialist proved this
As Wendy says, there's nothing to stop you registering with another vet anyway. Maybe instead of going to your normal vet you should just go to the new vet you want to visit and start this off there. I'd then just pop into the old vet and say please could you print out a history for me for my records - I wouldn't even mention another vet